The large volume of bug fixes is because a lot of people contributed
patches. Thanks!

Because you have given them a chance to do it :-)

Actually, with LDC or GDC that chance has been there for ages. The
compilable release of DMD makes it easier. I know I used GDC several
times over the years to debug and fix a number of issues as well as
develop and test a new feature.
Later,
Brad